The use of electrical and electronic equipment has multiplied rapidly in all fields in the last several decades. To a great extent, this growth has been fueled by the corresponding growth in the semiconductor industry which has provided ever more powerful electronic devices of ever decreasing size. The result has been the beneficial invasion of electrical and electronic devices into almost every area of life; home, office, work place, industry, laboratory, hospital, and school. An unintended consequence of the presence of electrical and electronic devices in such areas, however, is the potential that the electromagnetic emissions produced by the devices may be harmful to persons working in the immediate or general vicinity of those devices. Furthermore, newer devices often operate at high frequencies and contain relatively large circuits or amounts of circuitry in relatively small spaces.
Additionally, the devices themselves must also generally be protected from external interference of the same type; i.e., electrical or magnetic interference. Thus, most electronic devices are shielded for several purposes. Although protection of nearby personnel from electromagnetic emissions is one purpose of shielding, shielding is also used to reduce or prevent electrostatic coupling between the shielded item and other electronic items that may be either susceptible to or generators of electrostatic fields. Shielding can also improve performance of apparatus or test equipment by reducing losses, voltage gradients, or interference. Shielding is generally defined as a housing, screen or other object, usually conducting, that substantially reduces the effect of electric or magnetic fields on one side of the shield, and upon devices or circuits on the other side. Brief discussions of shielding can be found in almost any text or technical book of electronics or electrical equipment including the IEEE Standard Dictionary of Electrical and Electronics Terms, 1988, the Institute of Electrical and Electronics Engineers, Inc.
The most common method of shielding an electronic or electrical device is to place it inside of a conductive housing, as noted above, usually made of metal. From a practical standpoint, such housings or cabinets are typically and preferably formed in several parts which can be disassembled or otherwise opened to allow access to the electronic or electrical components inside. As a result, there exist joints or borders between and among the shielding components from which electromagnetic emissions can escape. Thus, in order to shield the boundaries between portions, and while permitting normal use and access, an electromagnetic shielding gasket material is typically used in a manner generally analogous to the manner in which a piece of weatherstripping prevents unwanted drafts from passing around a door or a window.
Early forms of such EMI gaskets consisted of stamped forms usually formed from metals such as Monel, copper, nickel, aluminum, and stainless steel. Although metals are highly conductive and thus provide excellent shielding properties, stamped metal gaskets exhibit some common disadvantages. First, they lack suppleness which prevents them from conforming to any unusual curvature or other shape in a shielding cabinet. Second, they require high closure pressures which in turn limits their use to very robust cabinets. Third, metal gaskets suffer permanent deformation when they are initially closed which tends to loosen subsequent closures between adjacent surfaces. Fourth, metal gaskets are often difficult to mount in a manner that will hold the gasket in place when the cabinet is opened.
Because of these and other disadvantages of stamped metal, other types of EMI gaskets have been developed. One alternative uses elastomeric materials as core elements under a conductive gasket sheath in an attempt to lower closure forces and improve suppleness. Other improvements include the direct attachment of the gasket to the boundary to be shielded using an adhesive, thus facilitating the gasket's attachment. A third type of improvement has replaced conductive wires or pure metals with silver plated yarn fabrics which tend to be somewhat more supple and resist permanent deformation from repeated closures better than the entirely metal gaskets.
Nevertheless, sheath and core gaskets, for example a elastomeric core with a conductive fabric covering, still exhibit certain disadvantages. In particular, cores formed from certain elastomers tend to suffer from too low a closure force--i.e., they deform too easily--with a corresponding lack of recovery to an original form after extended closure. Other elastomers suffer from the opposite problem of an unacceptably high closure force--i.e., they resist deformation too strongly--resulting from the attempt to reduce the "compression set" in the elastomer. To date, neither problem has been successfully addressed. Core materials such as polyurethane, for example, exhibit the low closure force that exhibits the corresponding lack of recovery problems. More resilient materials such as neoprene and silicone rubber will appropriately recover to their original form, but require overly high closure forces.
Such conventional gaskets exhibit other problems as well. These include a natural resistance against being bent into too sharp of a corner, torque bias, an inappropriately large distortion in volume or shape when placed in certain configurations, and other related problems.